In general, accompanying the market demand for miniaturization and weight-saving for electronic devices in recent years, the part items which constitute the devices are required to be thin-walled and lightweight.
For instance, electronic devices such as a capacitor, a laminated inductor element and the like that have heretofore been part items fitted with leads are made possible to be downsized through the practical application of a technique in which a monolithic structure equipped with an internal conductor is formed by simultaneously firing a ceramics layer having a prescribed pattern along with a laminate composed of an electroconductive layer. However, further miniaturization thereof is required at the present time.
A ceramic capacitor is usually produced by using a process comprising the steps of firstly preparing slurry by mixing ceramic powders having a high dielectric constant of a compound having perovskite-type crystalline structure such as barium titanate, a binder, an organic solvent and the like; applying coating of the slurry to a casting film made of polyethylene terephthalate or the like and drying the same to prepare a ceramic green sheet; then forming an electrode pattern on the resultant green sheet by means of screen printing or the like by using a conductive paste; peeling the ceramic green sheet off the casting film; subsequently laminating a large number of the printed ceramic green sheets in a prescribed order; bonding the same under heating and pressurizing; cutting the same into the form of desirable chips; subjecting the chips to firing treatment; and sintering the same.
On the other hand, the laminated inductor element is usually produced by using a process comprising the steps of firstly preparing a ceramic green sheet on a casting film by using magnetic ceramic powders such as ferrite in the same manner as the foregoing; then forming a coil pattern on the resultant green sheet by means of screen printing or the like by using a conductive paste; subsequently peeling the ceramic green sheet off the casting film; and then following the above-mentioned procedure to prepare the laminated inductor element in the form of chip.
The ceramic capacitor and the laminated inductor element each in the form of chip are required to be more and more miniature in order to cope with the demand for miniaturization as mentioned hereinbefore. Accompanying such demand, the ceramic green sheet, which has a thickness of 5 to 20 μm at the present time, is required to be thinner.
Since the previous casting film cannot cope with the green sheet which is made to have such a small thickness, a film having further high performances are necessitated, namely a casting film which is excellent in ceramic coating property and releasability from a ceramic green sheet and which has an extremely high flatness free from wrinkles and the like due to thermal shrinkage.
There has hitherto been generally used as a casting film, a polyethylene terephthalate film (PET film) which has been subjected to releasing treatment with a silicone resin based releasing agent of heat-curable addition reaction type. However, the silicone resin based releasing agent of heat-curable addition reaction type must be crosslinked usually at a high temperature of 140° C. or higher in order to assure a stable cured film. Consequently, wrinkles due to thermal shrinkage inevitably take place on a PET film in the releasing treatment step. Wrinkles due to thermal shrinkage, when being present on a PET film, bring about the problem in that the ceramic slurry cannot be made into a uniform thin-film sheet at the time of film forming.
Under such circumstances, in order to suppress the occurrence of the wrinkles due to thermal shrinkage to the utmost, an attempt is made to carry out the processing treatment with the silicone resin based releasing agent of heat-curable addition reaction type by lowering the processing rate under a condition of a lower temperature (110 to 130° C.). Nevertheless, the above-mentioned attempt gives rise to problems of not only inferior productivity but also insufficient curing, poor adhesion stability of the silicone resin for the PET film and poor coating property of the ceramic slurry.
In addition, a silicone resin based releasing agent of ultraviolet-alone-curable type which has a functional group such as an epoxy group, an acrylate group, a mercapto group or the like is known as a low temperature-curable silicone resin based releasing agent. However, the silicone resin based releasing agent just mentioned is difficult to assure uniform coated surface of the silicone resin and besides, is unstable and inferior in releasability of the ceramic green sheet.
In order to solve the above-mentioned problems, the present inventors had developed a casting film for producing a ceramic green sheet which casting film is obtained by applying coating of an addition reaction type silicone resin composition as a releasing agent containing a photosensitizer to a substrate film in a specific thickness, heat treating the coating at a prescribed temperature, and thereafter irradiating the same with ultraviolet rays {for instance, refer to Japanese Patent Application Laid-Open No. 198910/2001}.
Nevertheless, it has been proved that the film coated with a silicone resin has a high electrification property, thus causing various problems. For instance, surface electrification sometimes allows foreign matters to adhere thereto, thereby bringing about a defect or fault on the ceramic slurry to be applied onto the film. Moreover, film surface electrification sometimes causes fluctuation and repelling of the ceramic slurry to be applied thereto, thus failing to obtain a uniform thin film sheet. Further, such a problem is sometimes caused in that the slurry is disintegrated by defective peeling due to peeling electrification of a casting film for producing a ceramic green sheet in a step of peeling the ceramic slurry off the film.
Accordingly, importance has been attached in recent years to antistatic property of a casting film for producing a ceramic green sheet in addition to the above-mentioned adhesiveness to the substrate film, coating property of the ceramic slurry, releasability of the ceramic green sheet and a high flatness.